placing the same vnd underneath a vnd (with VNDIOCSET) is a lock violation,

placing the same vnd underneath a vnd (with VNDIOCSET) is a lock violation,
but other circumstances are also bad, so let's block all vnd on top of vnd.
While here, fix some toctou multiple-copyin of the path, and restructure
the ioctl defer all softc updates to the end.
ok mpi